Carbon dioxide cured sand molds employing dry sodium silicate binder



United States Patent 3,23%,099 CARBON lDliUXllDlE CURED SAND MQLDS EM- PL-GYZING DRY SGDIUM SlLlCATE BINDER Ronald H. Cooper, Clare, Mich., assignor to The Dow Chemical Company, Midland, Mich, a corporation of Delaware Filed Sept. l, 1960, Ser. No. 53,543 12 Claims. (Cl. 16--38.35)

The invention concerns an improved composition and method of use thereof for making sand molds and cores.

Cores and molds composed largely of sand intermixed with a binder are widely used and of extensive importance in the art of casting metals. A number of substances have been employed as binders for such use, among which is liquid sodium silicate. Different techniques for curing or hardening molds which are cast from compositions comprising sand and liquid sodium silicate have been tried among which is the use of carbon dioxide gas. Efforts to produce satisfactory sand molds for foundry use employing liquid sodium silicate as the inder, have not been fully satisfactory for a number of reasons among which are the sticky and messy nature of the composition during molding and the unsatisfactory immediate green strength (strength before cure) of molds made employing the known composition when cured by known methods including those employing carbon dioxide gas.

A need exists for a binder which does not have the sticky and otherwise undesirable characteristics of liquid sodium silicate which is employable with foundry sand, either in the presence or absence of such other known sand mold ingredients as pitch, invert sugar, sea coal, and dextrose compositions, and which is settable by subjection to the action of ambient carbon dioxide gas, to produce a mold or core having improved physical properties for subsequent use in the casting of metals.

The invention meets this need by providing the improved composition and method employing such composition hereinafter described and defined.

The invention, accordingly, is a moldable deformable composition of matter comprising foundry grade sand, dry sodium silicate consisting of between about 0.5 and 1.5 moles of SlGg per mole of N320, and sufiicient water to make a free flowing moldable mass of desired consistency and the method of making molds and cores comprising admixing the sodium silicate as a dry powder with the sand, admixing water therewith to make a moldable composition, forming the composition into desirable shapes, and curing or setting the shapes or castings by subjecting them to the action of a C0 atmosphere for a short time, e.g., 5 seconds to a minute. No baking or heating during the C0 treatment is required. The molds or cores made according to the invention are readily collapsible after use in molding operations. The cores or molds made according to the invention, characteristic of freshly made sand molds generally, may have the compressive strength improved by aging, i.e., standing or remaining in storage for a time before use. Molds made according to the invention show improvement up to at least about 24 hours after the CO treatment. Although one of the advantages of the invention is the lack of any need for ovens or heating equipment, such equipment when at hand may be employed to effect the benefits of aging within a shorter time. For example, minutes at about 325 F., following CO treatment, effects about the same improvement in compressive strength as 24 hours aging at room temperature. In practice, however, it usually entails less inconvenience to set aside the molds or cores for a few hours at room temperature than to be encumbered by heating ovens (otherwise not needed) and undergo the added cost and bother of accelerating aging by heating.

Patented Jan. 18, 1966 "ice A carbonaceous substance, e.g., pitch, sea coal, or dcxtrin, when admixed with the dry sodium silicate and sand mixture and subsequently water-wetted has been found to improve the properties thereof and such practice constitutes an embodiment of the invention.

When the molar ratio of SiO to Na O in the dry sodium silicate approaches or is 0.5, at least 2 parts thereof, per 100 parts by weight of the sand, should be em ployed. When sodium silicate, having an increased molar ratio of Si0 to the Na O is employed, the minimum amount of such sodium silicate to employ increases somewhat so that when the molar ratio of SiO to Na O approaches or is 1.5, at least 3 parts per 100 parts by weight of sand should be employed. Not over 6 parts of sodium silicate, per 160 parts by weight of the sand is usually employed in the invention. The annexed figure shows graphically the parts by weight sodium silicate having varying ratios of SiO to Na O, per 100 parts of sand, employed in the invention.

The amount of water employed is substantially the same as the weight of the sodium silicate employed although somewhat mo-re or less water may be employed. For example, best results are obtained by employing an amount of water which is somewhat greater than the amount of sodium silicate when the sodium silicate employed contains the higher acceptable molar ratio of SiO to Na O, viz. that of 1.5.

When sodium disilicate, sometimes expressed by the formula, Na Si O having a molar proportion of 2 SiO 1 Na O, is employed in a sand mixture containing the desired moisture content, the composition so made does not set satisfactorily upon subsequent treatment with C0 gas. The preferred sodium silicate to use is that commonly referred to as orthosilicate, sometimes expressed by the formula, Na SiO which is a molar ratio of SiO to Na O of 0.5. It is preferably employed in an amount of between 2 and 4 parts per 100 parts by weight of sand.

The sand employed is foundry grade sand having an A.F.S. number of between 50 and 150. A grade between No. 60 and is preferred. Details on A.F.S. foundry grade sand and the significance of the numbers are procurable from the American Foundrymens Society, Chicago, Illinois, U.S.A. They are discussed in Foundry Core Practice, by Harry W. Dietert, published by the American Foundrymens Society (1952).

Curing of the molded or formed sodium silicate-sandwater composition thus made is effected in any con venient chamber provided with suitable means for in troducing CO gas. The length of time required for a satisfactory cure depends upon a number of factors, chief among which is the thickness of the molds and the rate of introduction of CO gas. A CO line gauge pressure of about 14 pounds per square inch is commonly employed, but less or greater pressures are suitable. The length of time required for cure varies from about 5 to seconds, 5 to 20 seconds usually being employed when using a sodium silicate having a molar ratio of SiO to Na O of 1.5 and 40 to 60 seconds usually being employed for a sodium silicate mole ratio of 810 to N520 of 0.5. Descriptions of ovens suitable for use in curing with carbon dioxide gas and details of their operations are described in A Practical Guide to the CO Process, published jointly by the Textile Chem ical Co. and the International Foundry Supply Co. and Handbook of the Carbon Dioxide Mold and Core Hardening Procedure, procurable from the Carver Foun dry Products Co., Muscatine, Iowa.

Bank sand was employed because it offers more difficulties than other foundry grade sand, due to the presence of between 0.5 and 1.0 percent clay. However,

it is more readily procured and less expensive. Its suc In Comparative Run C, only 20 grams (2 percent by cessful use is a more severe test of the suitability of the weight) of sodium silicate having an acceptable molar composition of the invention and also indicates the exratio of SiO to Na O of 1.5 were used. tent of its economic significance. In Examples 7 to 15, the sodium silicate employed The molds of cores Produced in accordance with the was that having an acceptable molar ratio of SiO to invention have good compressive strength values. For o of 05 representing the minimum limit of the purposes of illustrating the invention, the cores were range of sioz to Nazo satisfactory for the practice of formed in a Harry D. Dietert Company (Detroit, Michigan) master precision specimen tube, 2" x 4%" in size, employing core box No. 315-18, and rammed to a 2" 10 length by three successive impacts of a Harry W. Dietert the invention. The amounts of both the sodium silicate and water were varied from 20 to 40 grams (2 percent to 4 percent by weight). 10 grams (10 percent by weight) sand rammer No. 315-17. The cores thus formed, after Pitch were added In Examples 3,11,

Additament to core Amount added in grams Compressive Run dosignacomposition, mole strength in p.s.i.

tion ratio of powdered after air-drying 24 sodium silicate Silicate H Pitch hours following 001 treatment A 2 Si02:1 NazO 40 40 2 810211 NazO. 40 60 1.5 SiOgzl Naz 40 40 768 1.5 SiOzzl Na2O 40 50 982 1.5 SiO2:1 NazO 40 50 1,035 1.5 SiOzZl NazO 30 380 1.5 SiOazl NazO 30 070 1.5 SiOzzl Na2O 30 40 480 1.5 SiOzzl NazO 20 20 67 0.5 810221 Na2O 40 40 1,030 0.5 SiOzzl NazO 40 40 926 0.5 SlOzZl NazO 30 30 568 0.5 810221 NazO 30 40 354 0.5 SiOzzl NazO 30 30 600 0.5 S1022]. NazO 30 40 408 0.5 SiOzzl Na2O 20 20 294 0.5 SiO-nl NazO 20 30 206 0.5 siom Na20- 20 20 249 1 Will not gell with CO1.

1000 grams of N0. 67 A.F.S. sand used in all runs.

Mulling time: 1 minute before adding water and 3 minutes alter water added, in all runs. Curing conditions: 10-60 seconds 111 ambient 00; gas.

cure i bi t CO gas, were tested in the Harry W. Referring to Comparative Runs A and B of the table, Dietert Universal Sand Strength Machine to obtain com- 1t can readily be seen that where. the molar r t of h pressive strength values in p.s.i. Compressive strength 2 t0 N z 111 sodlulfl slllcate 1S a empts to pr0- values of less than about 200 p.s.i. or more than about dllce Satisfactory cores far-l.

1200 p.s.i., as obtained by the Dietert machine, are con- 40 Examples 1 to show that satisfactory cores are prosidered unsatisfactory for the manufacture of cores. dllced W t $10 0 Na O 1n the sodium silicate em- The following test runs were made, some of which are P y has molar ratio of when p y in an illustrative of the practice of the invention, designated amount varying between 3 and 4 percent, by Weight of examples, and some not illustrative of the invention, e Sand and ln the presence of between 3 and 5 perdesignated comparative runs. The examples are discent water, with or withoutpitch being added. tinguished by numbers and the comparative runs by comparative Run C, which employs 2 percent of solettem drum silicate having a molar ratio of SiO to Na O of In each of the runs, 1000 grams of A.F.S. No. 67 Gra- 1.5 (which is satisfactory in higher amounts) was shown tiot bank foundry grade sand and dry sodium silicate, to produce an unsatisfactory core when such molar ratio having the molar ratio of Si0 to M1 0 and in the 1s employed in that amount. amount stated in the table below, were put in a muller Examples 7 to 15 show that sodium silicate having a and intermixed for 1 minute. Then the amount of water molar ratio of SiO to Na O of 0.5 can be employed in stated in the table was added thereto and mixed for 3 an amount of between 2 and 4 percent by weight of the minutes. A free-flowing moldable composition was sand, in the presence of between 2 and 4 percent water, thereby produced which was cast into the 2-inch long and with or without pitch, to produce satisfactory cores. cylindrical test cores described above and cured in a It also, therefore, shows that when the ratio of SiO; chamber into which CO gas was admitted at a pressure to Na O approaches the minimum of the permissible ratio of 14 pounds/ square inch gauge pressure for between 10 specified in the practice of the invention, cores can be and seconds. The cores so made were removed made employing only 2 percent sodium silicate. therefrom and allowed to age 24 hours at room tempera- 60 Reference to the annexed figure graphically shows the ture and thereafter the compression strength values thereparts of sodium silicate, having the desired siO zNa O of determined by use of the Dietert testing machine deratio to employ in parts by weight per 100 par-ts of sand. scribed above. The more important details and test Tests were conducted to ascertain the collapsibility of results are entered in Table I which is set out hereinafter. the cores produced in accordance with the invention and In Comparative Runs A and B, 40 grams (4 percent contrasted with cores produced by known practice. The by weight) of sodium silicate having a molar ratio of cores employing the dry powdered silicate in accordance SiO to Na O of 2 was used. 40 grams of water were with the invention showed equally satisfactory coladded in Run 1 and 60 grams in Run 2. lapsibility to those made in accordance with known prac- In Examples 1 to 6, the sodium silicate employed was tice. that having an acceptable molar ratio of SiO to Na;() An examination of the examples and comparative runs of 1.5 and was varied in the amount of either 30 or 40 set out in the table shows that the composition of the grams (3 percent or 4 percent by weight). The water invention prepared and cured in accordance with the was varied between 30 and 50 grams, (3 to 5 percent by method of the invention produces a core which is comweight). In Examples 3 and 6, 10 grams of pitch were parable or superior to cores produced employing known admixed therewith. 7 5 compositions without the accompanying disadvantages of using a sticky objectionable mixture which exists when liquid sodium silicates or solutions thereof are employed.

Having described the invention, what I claim and desire to protect by Letters Patent is:

1. A composition of matter useful for making sand cores and molds curable by subjecting to an ambient atmosphere comprising carbon dioxide gas consisting of a mixture of foundry grade sand and a powdered dry sodium silicate, composed of between 0.5 and 1.5 moles of SiO per mole of Na i), and in an amount of at least 2 parts of the sodium silicate when the molar ratio of S10 to Na O approaches 0.5 and in an amount of at least 3 parts per 100 parts of sand, when the molar ratio of SiO to Na O approaches 1.5, and water in an amount sutficient to make a moldable mass but not substantially in excess of the weight of the sodium silicate employed.

2. The composition of claim 1 wherein the dry sodium silicate is composed of about 0.5 mole of SiO per mole of Na O and the amount of water employed is substantially equal to the weight of the sodium silicate employed.

3. The composition of claim 2 wherein the dry sodium silicate is employed in an amount of between 2 and 4 parts per 100 parts of said sand.

4. The composition of claim 2 wherein the sand employed has an A155. number of between 60 and 80.

5. A composition of matter comprising foundry grade sand, powdered sodium silicate having the molar ratio and in the amount per 100 parts by weight of the sand defined by the preferred operable area shown in the annexed drawing, and water in a sufficient amount to make a freedlowing moldable mass which sets to a unitary solid suitable for molding purposes when subjected to a C gas atmosphere.

6. The composition of claim 5 which contains up to about 1 percent, by weight of said sand, of a carbonaceous substance selected from the class consisting of pitch, sea coal, dextrin, and mixtures thereof and water in sufficient amount to make a moldable mass.

7. The method of making sand cores and molds consisting of admixing, in the dry state, foundry grade sand, a powdered sodium silicate composed of between 0.5 and 1.5 moles of SiO per mole of Na i) in an amount of at least 2 parts per 100 parts by weight of the sand when the molar ratio of Si0 to Na G approaches 0.5 and in an amount of at least 3 parts per 100 parts of sand when the molar ratio of SiO to Na O approaches 1.5, and sufficient water to make a moldable mass but not substantially in excess of the weight of the sodium silicate used, molding the composition thus made into a desirable shape, and subjecting the molded shape to an ambient atmosphere comprising carbon dioxide gas to effect a cure thereof.

8. The method according to claim 7 wherein the sodium silicate employed consists of about 0.5 mole of SiO per mole of N-a O and the amount of water employed is substantially the weight of the sodium silicate employed.

9. The method according to claim 3 wherein the sodium silicate is admixed with the sand and water in an amount of between 2 and 4 parts per parts by weight of sand.

it). The method according to claim 8 wherein the sodium silicate admixed with the sand and water has the molar ratio of Si0 to Na O and in the amount in parts per 100 parts by weight of the sand defined by the preferred operable area shown in the annexed drawing.

11. The method according to claim 3 wherein up to 1 percent, by weight of the sand, of a carbonaceous substance selected from the class consisting of pitch, sea coal, dextrine, and mixtures thereof is admixed with the sand, sodium silicate, and water.

The method according to claim 8 wherein the ambien-t carbon dioxide atmosphere is sufficiently concentrated to effect a cure of the molded shapes in not over about 1.5 minutes.

References Qited by the Examiner UNITED STATES PATENTS 464,367 1/1891 Kimble 106-84 2,128,404 8/1938 Dunbeck 106-383 2,874,428 2/1959 Bonney 106-3855 2,883,723 4/1959 Moore et a1. 106-383 2,905,563 9/1959 Ilenda et al 106-3835 2,952,553 9/1960 llenda et al 106-383 FGREIGN PATENTS 710,099 6/1954 Great Britain.

OTHER REFERENCES Schumacher: Carbon Dioxide Process for Baking Molds and Cores, American Foundry Man, September 1954 (pages 46-49).

ALEXANDER H. BRODMERKEL, Primary Examiner.

JOHN R. SPECK, JOSEPH REBOLD, MORRIS LIEBMAN, Examiners. 

1. A COMPOSITION OF MATTER USEFUL FOR MAKING SAND CORES AND MOLDS CURABLE BY SUBJECTING TO AN AMBIENT ATMOSPHERE COMPRISING CARBON DIOXIDE GAS CONSISTING OF A MIXTURE OF FOUNDRY GRADE SAND AND A POWDERED DRY SODIUM SILICATE, COMPOSED OF BETWEEN 0.5 AND 1.5 MOLES OF SIO2 PER MOLE OF NA2O, AND IN AN AMOUNT OF AT LEASE 2 PARTS OF THE SODIUM SILICATE WHEN THE MOLAR RATION OF SIO2 TO NA2O APPROACHES 0.5 AND IN AN AMOUNT OF AT LEAST 3 PARTS PER 100 PARTS OF SAND, WHEN THE MOLAR RATIO OF SIO2 TO NA2O APPROACHES 1.5, AND WATER IN AN AMOUNT SUFFICIENT TO MAKE A MOLDABLE MASS BUT NOT SUBSTANTIALLY IN EXCESS OF THE WEIGHT OF THE SODIUM SILICATE EMPLOYED. 